Harnessing the Versatility of Continuous-Flow Processes: Selective and Efficient Reactions

There is a great need for effective transformations and a broad range of novel chemical entities. Continuous-flow (CF) approaches are of considerable current interest: highly efficient and selective reactions can be performed in CF reactors. The reaction setup of CF reactors offers a wide variety of possible points where versatility can be introduced. This article presents a number of selective and highly efficient gas-liquid-solid and liquid-solid reactions involving a range of reagents and immobilized catalysts. Enantioselective transformations through catalytic hydrogenation and organocatalytic reactions are included, and isotopically labelled compounds and pharmaceutically relevant 1,2,3-triazoles are synthesized in CF reactors. Importantly, the catalyst bed can be changed to a solid-phase peptide synthesis resin, with which peptide synthesis can be performed with the utilization of only 1.5 equivalents of the amino acid.

Electrophilic Aromatic Deuteration of Lignans: Mostly Reliable but Occasionally Aberrant Selectivities

Lignans are a ubiquitous group of natural products of plant or mammalian origin. In the human diet, especially in fiber-rich foods, there are measurable amounts of lignans. Lignan intake is associated with a reduced risk of a range of chronic Western diseases, and in studying these compounds and their biological activity, authentic stable isotope labeled analogues are needed. This review summarizes the reported labeling methods and discusses the selectivity and reactivity in the electrophilic aromatic deuteration of lignans where recently a number of unexpected selectivities or nonselectivities have been encountered.

High-resolution twin-ion metabolite extraction (HiTIME) mass spectrometry: nontargeted detection of unknown drug metabolites by isotope labeling, liquid chromatography mass spectrometry, and automated high-performance computing

The metabolic fate of a compound can often determine the success of a new drug lead. Thus, significant effort is directed toward identifying the metabolites formed from a given molecule. Here, an automated and nontargeted procedure is introduced for detecting drug metabolites without authentic metabolite standards via the use of stable isotope labeling, liquid chromatography mass spectrometry (LC/MS), and high-performance computing. LC/MS of blood plasma extracts from rats that were administered a 1:1 mixture of acetaminophen (APAP) and (13)C6-APAP resulted in mass spectra that contained "twin" ions for drug metabolites that were not detected in control spectra (i.e., no APAP administered). Because of the development of a program (high-resolution twin-ion metabolite extraction; HiTIME) that can identify twin-ions in high-resolution mass spectra without centroiding (i.e., reduction of mass spectral peaks to single data points), 9 doublets corresponding to APAP metabolites were identified. This is nearly twice that obtained by use of existing programs that make use of centroiding to reduce computational cost under these conditions with a quadrupole time-of-flight mass spectrometer. By a manual search for all reported APAP metabolite ions, no additional twin-ion signals were assigned. These data indicate that all the major metabolites of APAP and multiple low-abundance metabolites (e.g., acetaminophen hydroxy- and methoxysulfate) that are rarely reported were detected. This methodology can be used to detect drug metabolites without prior knowledge of their identity. HiTIME is freely available from https://github.com/bjpop/HiTIME .

Quantitative metabolomic profiling using dansylation isotope labeling and liquid chromatography mass spectrometry

Differential chemical isotopic labeling (CIL) LC-MS has been used for quantifying a targeted metabolite in biological samples with high precision and accuracy. Herein we describe a high-performance CIL LC-MS method for generating quantitative and comprehensive profiles of the metabolome for metabolomics applications. After mixing two comparative samples separately labeled by light or heavy isotopic tags through chemical reactions, the peak intensity ratio of the labeled analyte pair can provide relative or absolute quantitative information on the metabolites. We describe the use of (12)C2- and (13)C2-dansyl chloride (DnsCl) as the isotope reagents to profile the metabolites containing amine and phenolic hydroxyl functional groups by LC-MS. This method can be used to compare the relative concentration changes of hundreds or thousands of amine- and phenol-containing metabolites among many comparative samples and generate absolute concentration information on metabolites for which the standards are available. Combined with statistical analysis and metabolite identification tools, this method can be used to identify key metabolites involved in differentiating comparative samples such as disease cases vs. healthy controls.

Peptide and protein drug analysis by MS: challenges and opportunities for the discovery environment

Straightforward assay development using MS has become commonplace in most modern pharmaceutical laboratories. In particular, MS is an invaluable tool in the discovery environment of this industry, making it possible to characterize the structures of target drugs and to screen large numbers of potential drug candidates in metabolism and pharmacokinetics studies, and much more. Furthermore, as drug portfolios expand to include biotherapeutic species, such as peptides and proteins, MS is there to meet any analytical challenges. In this article, general aspects of MS in the discovery environment are discussed, as well as what the future might hold.

Critical topics in ensuring data quality in bioanalytical LC–MS method development

The use of LC–MS for bioanalysis of pharmaceuticals is entering its third decade and may be considered to be a mature technology. In many respects this is true, considering the advances made in such areas as instrument performance, electronics, software and automation of use. However, there remain instrumental and noninstrumental areas that require significant attention to ensure data quality. Increasing regulatory focus on analytical method performance and unaddressed method issues require the bioanalyst to understand those areas that most greatly impact data quality. This review will focus on instrumental and noninstrumental areas that can influence data quality, including reference standard and internal standard quality and physicochemical properties, matrix effects, stability in matrix, sample preparation, LC and MS.

Application of stable isotope-labeled compounds in metabolism and in metabolism-mediated toxicity studies

Stable isotope-labeled compounds have been synthesized and utilized by scientists from various areas of biomedical research during the last several decades. Compounds labeled with stable isotopes, such as deuterium and carbon-13, have been used effectively by drug metabolism scientists and toxicologists to gain better understanding of drugs' disposition and their potential role in target organ toxicities. The combination of stable isotope-labeling techniques with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy, which allows rapid acquisition and interpretation of data, has promoted greater use of these stable isotope-labeled compounds in absorption, distribution, metabolism, and excretion (ADME) studies. Examples of the use of stable isotope-labeled compounds in elucidating structures of metabolites and delineating complex metabolic pathways are presented in this review. The application of labeled compounds in mechanistic toxicity studies will be discussed by providing an example of how strategic placement of a deuterium atom in a drug molecule mitigated specific-specific renal toxicity. Other examples from the literature demonstrating the application of stable isotope-labeled compounds in understanding metabolism-mediated toxicities are presented. Furthermore, an example of how a stable isotope-labeled compound was utilized to better understand some of the gene changes in toxicogenomic studies is discussed. The interpretation of large sets of data produced from toxicogenomics studies can be a challenge. One approach that could be used to simplify interpretation of the data, especially from studies designed to link gene changes with the formation of reactive metabolites thought to be responsible for toxicities, is through the use of stable isotope-labeled compounds. This is a relatively unexplored territory and needs to be further investigated. The employment of analytical techniques, especially mass spectrometry and NMR, used in conjunction with stable isotope-labeled compounds to establish and understand mechanistic link between reactive metabolite formation, genomic, and proteomic changes and onset of toxicity is proposed. The use of stable isotope-labeled compounds in early human ADME studies as a way of identifying and possibly quantifying all drug-related components present in systemic circulation is suggested.